It is known that diversity can be used to increase the signal level from a mobile phone to a base station (uplink). Diversity is applied on the reception side of the base station. A transmitted signal extremely rarely reaches the user via the most direct route. The received signal is very often a combination of direct and reflected electromagnetic waves. The reflected waves have differing phase and polarization characteristics. As a result, there can be an amplification or, in extreme cases, cancellation of the signal at specific locations. Operation in a canyon-like street, e.g., is often only possible by using reflections. These reflections from buildings, masts or trees are common, because mobile communications predominantly uses vertical polarization.
A space diversity system is known to consist of two reception antennas spaced a distance apart. One antenna has a certain field strength profile with maximums and minimums from its coverage area; the other antenna has a different field strength profile although only spaced a few meters away. Ideally, the minimum of one antenna will be completely compensated by the maximum of the other. The improvement in the average signal level achieved with this method is called diversity-gain. Both antennas function separately on different reception paths, whereby the higher signal per channel and antenna is chosen by the base station. Separation in the horizontal plane is often used (horizontal diversity). The results of vertical diversity are known to be worse.
A typical GSM Omni-base Station is made up of 3 antennas: one transmitting antenna (Tx) and two receiving antennas (Rx). The transmitting antenna is usually mounted higher and in the middle of the two receiving antennas in order to guarantee a cleaner omni-directional characteristic. Furthermore, the influence of the Rx and Tx antennas on each other is reduced (higher isolation). The two receiving antennas are usually spaced at 12-20 lambda to achieve a diversity gain of 4-6 dB.
Omni-base stations are mainly installed in regions with a relatively low number of subscribers. For capacity reasons, the communications cell is divided into 3 sectors of 120° in urban areas. Directional antennas, for example panels, are used to cover these sectors. All 3 antennas per sector can be mounted at the same height because directional antennas have higher isolation in comparison to omni-directional antennas.
The reflections which take place within urban areas are not all of the same polarization, i.e. horizontal components also exist. Furthermore, a mobile telephone is never held exactly upright which means that all polarizations between vertical and horizontal are possible. Therefore, known systems also use these signals. Space diversity uses 2 vertically polarized antennas as reception antennas and compares the signal level. Polarization diversity uses 2 orthogonally polarized antennas and compares the resulting signals. The dipoles of both antenna systems are horizontally and vertically polarized, respectively. A spatial separation is not necessary which often results in the differently polarized dipoles being mounted in a common housing. As a result, in known systems, 2 antennas can be sufficient per sector: 1×hor./vert. for polarization diversity, 1×vert. for Tx. If, in addition, the vertical path of the dual polarized antenna is fed via a duplexer for Rx and Tx, then only one antenna is needed per sector. As a result, all 3 sectors can be supplied from one mast. The diversity gain in urban areas is the same as that achieved via space diversity (4-6 dB).
It is also possible to use dipoles at +45°/−45° instead of horizontally and vertically (0°/90°) placed. It is known that this creates the possibility of two identical systems being able to handle both horizontally and vertically polarized components. Two transmitting channels using hor/ver antennas can be combined via a 3-dB-coupler onto the vertical path. As a result, half the power of both transmitting channels will be lost. Both polarizations are known to be suitable for Tx if cross-polarized antennas are used.
In mobile networks, it is common to use antennas that create sector-shaped beams. To build a smaller sector, i.e. creating a smaller beam width, two antennas can be connected to achieve half the beam width. Conventionally, if two of such small sectors are necessary, the two antennas have to be connected for each sector, i.e. two times, thus quadrupling the total antenna space.
German patent application DE10116964 discloses an antenna structure for polarization diversity reception with four antennas fitting together in a dimensional perspective with different polarization/orientation.
U.S. Pat. No. 6,583,763 discloses an antenna structure and installation. A distributed antenna array includes a plurality of antenna elements and a plurality of power amplifiers, each power amplifier being operatively coupled with one of the antenna elements and mounted closely adjacent to the associated antenna element, such that no appreciable power loss occurs between the power amplifier and the associated antenna element.
U.S. Pat. No. 3,979,754 discloses a radio frequency array antenna employing stacked parallel plate lenses. A radio frequency multi-beam array antenna is disclosed wherein a beam-forming network includes a first set of vertically disposed parallel plate lenses coupled between a matrix of radiating elements and a second set of horizontally disposed parallel plate lenses. With such a beam forming network, a plurality of narrow pencil-shaped beams of radiation may be formed over a relatively large solid angle.
US patent application US2004/0014502 discloses an antenna system for a transmitter comprising an array of antennas and control means. The antennas are arranged to transmit over all or part of the coverage area of the transmitters. The control means control the number of antennas that are used to transmit a signal in dependence on the width of the signal to be transmitted.
U.S. Pat. No. 6,195,063 discloses a dual-polarized antenna system. A dual-polarized antenna system is provided for transmitting or receiving electromagnetic waves. The antenna system has at least one cruciform radiating element module that is aligned using dipoles or in the form of a patch radiating element, at angles of +45° and −45° with respect to vertical. The antenna system further has a conductive reflector arranged in the back of the at least one radiating element module. Two conductive sidewall sections are provided on each side of the at least one radiating element and are disposed vertically. At least one slot is provided in each sidewall section at the level of the radiating element module and extends in parallel to the reflector plane.
Problem Definition
The prior art fails to disclose a solution for generating and utilizing several small beams from several wide-beam antennas without having to double the amount of antennas per beam.
Aim of the Invention
The aim of the invention is to generate and utilize several small beams from several wide-beam antennas, using only a fraction of the antenna space as conventionally needed.